Signal-enhancement system for photodetector outputs

ABSTRACT

A light scattering photometer signal-enhancement systems includes an adaptive sample and subtract circuit controlled by a computer or microcontroller (MCU). The MCU controls the gain of a programmable-gain amplifier (PGA) cascade that is used to amplify the raw photometer signal. In order to maintain the DC accuracy, the DC offset contained in the raw signal from the photometer is estimated by an algorithmic within the MCU and then subtracted from the raw signal before allowing it to be amplified by the PGA cascade. In addition to DC estimation and adaptive cancellation, the MCU applies a digital filtering scheme to compensate irrelevant frequency bands in the amplified signal and offers user determined averaging functions for additional signal conditioning. Moreover, hardware filters are used to prevent signal aliasing by the analog to digital converters (ADC) and a 60 Hz notch filter suppresses general electrical noise.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of Ser. No.11/255,875, filed Oct. 24, 2005.

BACKGROUND OF THE INVENTION

The field of the invention is signal enhancement of photo detectoroutputs as seen, for example, in aerosol particle detection andmeasurement systems. In particular, the present invention is related toenhancement of electrical signals for the detection of light detected bya photodiode and more specifically by light detected by a photodiode asthe result of light scattering from a concentration of aerosolparticles.

Examples of systems in the background art that use photodetectorsinclude, but are not limited to, optical character recognition systems,communication systems medical imaging sensors, laser range finders,radiation detectors, smoke detectors, position sensors and proximitysensors. In all of these background art systems, a photodetector is usedto measure light or radiation in terms of an electrical signal that isprocessed in various ways to produce a useful information output. In aparticular example from the background art, a beam of collimated light,which may or may not be coherent, is directed through a transparent cellin which particles suspended in fluid mixtures are made to pass.Photodetectors are then used to detect the relative amount of light thatis scattered or blocked by the particles. The signals generated by thephotodetector contain information about the concentration of particles,size of particles, and/or presence of particles.

The type of photodetector used depends on the sensitivity requirementsof the device. A photo-multiplier tube is the most sensitive (andcostly) method that is currently available. A photo-multiplier candetect the presence of a single photon with nanosecond resolution.However, photo-multiplier tubes are very costly to manufacture and areeasily damaged. Additionally they have very high voltage requirementsand therefore tend to be used in laboratories rather than in commercialapplications.

One alternative to using a photo-multiplier tube is to use a photodiodeand a transimpedance amplifier. In contrast to a photo-multiplier,photodiodes are inexpensive, rugged, small, and operate at low voltages.

Another background art device that is used to measure aerosol particlesize and concentration is called a light scattering photometer ornephelometer. Applications that require particularly sensitivemeasurements require photo-multiplier-based photometers.

When the sensitivity requirements of the application do not justify theuse of a photo-multiplier tube, a photodiode-based device is preferreddue to the reduced cost. However, background art photodiodes are not assensitive as photo-multiplier tubes and are prone to noise problemsassociated with electrical amplification.

The sensitivity of a photodiode device is in part a function of the gainof a transimpedance amplifier associated with the photodiode device. Theamplified signals contain useful information pertaining to the amount oflight reaching the photo detector. However, due to the inherentproperties of the photo detector and amplifier circuits, the amplifiedsignal also contains additional factors such as offset voltagepotential, noise generated by ambient light and electromagneticinterference. These additional factors have the effect of limiting thepossible gain of the amplifier stages before reaching saturation.Therefore, there is a need in the art for a low-cost photodiode-baseddetector with improved gain and sensitivity.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for amplification,filtering, DC cancellation, and signal processing of photodetectoroutput signals in order to extract useful information related to theamount of light reaching the detector. Examples of photodetectorsinclude, but are not limited to, photodiodes, phototransistors,photomultiplier tubes and Charge Coupled Device (CCD) image sensors. Thepresent invention also provides a means for outputting the usefulinformation extracted from the photodetectors via at least one of aserial link, visual display, analog output, radio link, or audio output.

In the present invention, the gain and sensitivity of a photodiode-baseddetector is increased by at least removing the noise and DC offset. Thisincreased sensitivity allows the photodiode-based detector of thepresent invention to be used in applications that currently require aphoto-multiplier-based photometer.

One embodiment of the present invention is an apparatus for enhancingelectronic signals from a photodetector comprising: an amplifier; aclamp circuit connected to the amplifier; a programmable gain amplifierconnected to the clamp circuit; filter connected to the programmablegain amplifier; a notch filter connected to the filter; ananalog-to-digital converter connected to the notch filter; at least onedigital-to-analog converter connected to the analog-to-digitalconverter; an inverting-summing amplifier; an input of the amplifier; atleast one DC reference generator; and a computer, wherein the computeris connected to the analog-to-digital converter, the programmable gainamplifier, the filter, the notch filter and the at least onedigital-to-analog converter and provides feedback control and digitalfiltering for the apparatus.

Another embodiment of the present invention is a method for enhancingelectronic signals from a photodetector comprising: at least one ofstarting and resetting the photodetector; initializing digital-to-analogconverters (DACs), analog-to-digital converters (ADCs) programmable gainamplifiers (PGAs) and filter parameters; subtracting a voltage incrementfrom the output of a filter until the output of the filter is at leastone of less than and equal to a predetermined coarse threshold voltage;subtracting a voltage increment from the output of the filter until theoutput of the filter is at least one of less than and equal to apredetermined fine threshold voltage; filtering and signal processingthe output of the filter; and outputting the filtered and signalprocessed output of the filter until receiving at least one of a powerdown and reset command.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be described in greater detail with the aid of thefollowing drawings.

FIG. 1 is an exemplary block diagram showing the functional blocks usedto implement the apparatus and method of the present invention.

FIG. 2 is an exemplary flowchart showing the photometer signal autozeroing via an adaptive DC cancellation algorithm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The exemplary system block diagram of FIG. 1 shows the apparatus of thepresent invention. In particular, FIG. 1 shows an example of thefunctions of amplification, filtering, DC-cancellation, and signalprocessing of a photometer device. The analog signal from the photometeris input into the apparatus via a twisted pair cable connected to thenon-inverting input of an instrumentation amplifier 1. The amplifier 1can be any type of instrumentation amplifier and should be selected witha high common mode rejection ratio (CMRR) as the major deciding factor.The amplifier 1 should ideally have a CMRR of at least 85 dB. Thisamplifier 1 in the system can also provide a small gain to the signal(i.e., between 1 and 10). The output of the instrumentation amplifier 1may then pass through a voltage clamp 2 to protect the rest of thesystem from over-voltage or under-voltage signals.

After passing through the voltage clamp 2, the analog signal passesthrough at least one programmable gain amplifier (PGA) 3 that furtheramplifies the signal. The exact gain of the PGA 3 is controlled by themicrocontroller (MCU) 7, and can be programmed to suit the specificapplication. In addition, the gain may be static or a function of acontrol algorithm.

The signal is then filtered with a low pass filter 4 with a cutofffrequency that may be fixed or controlled by the MCU 7. The low passfilter 4 may be passive or active and may be activated or deactivated bythe MCU 7, or bypassed with the use of a jumper. A notch filter 5 isthen implemented to remove frequency specific noise in the signal. Thestop band of the notch filter 5 has a default frequency of 60 HZ, andcan be shifted by the MCU 7 or with settings determined by jumpers. Thenotch filter 5 may also be passive or active, and may be bypassed by theMCU 7 or with the use of a jumper. The output of the PGA 3 and filterblocks 4, 5 may be provided as an analog output 11 of the apparatus.

The MCU 7 will receive this filtered signal after the filtered signalpasses through an internal or external analog-to-digital converter (ADC)6. The MCU 7 will also control several digital to analog converters(DACs) 8, 9, which may be internal or external to the MCU 7. The voltagereferences of the DAC blocks may be set so that each one is lower than aprevious voltage reference. This configuration allows for a courseadjustment DAC 8, and successively finer adjustments DAC 9.

An inverting summing amplifier 10 is used to sum and invert the outputsof one or more DACs 8, 9. This inverted sum is then input to theinstrumentation amplifier 1 to create a negative DC offset for DC signalcancellation.

Optionally, another DAC (not shown) may be included in the output block11 and would be controlled by the MCU 7. This enables the MCU 7 toimplement a multitude of digital filtering techniques and to output theresult as an analog voltage. The MCU 7 can also control an internal orexternal serial port or other device for serial output. Any number ofother output devices may be driven by the MCU 7 to provide an audiooutput, visual display, or radio link output.

In the apparatus discussed above, preferably the photodetector has amaximum sensitivity having an approximate wavelength of between at leastone of 100 and 400 nm, 400 and 600 nm, 600 and 700 nm, 700 and 1100 nmfor the ultra violet spectrum, blue-green-yellow spectrum, red spectrum,and infrared spectrum, respectively.

Further, in the apparatus discussed above, preferably a signal from thephotodetector is amplified via a trans-impedance amplifier to achieve again of between at least one of 1 and 30,000; 1 and 10,000,000, whereinthe amplifier has a common mode amplification is achieved from aninstrumentation amplifier with a high Common Mode Rejection Ratio (CMRR)and a gain of5.

Further, in the apparatus discussed above, the instrumentation amplifierpreferably has at least one of a common mode amplification gain that isvariable between at least one of 1 and 100; a common mode amplificationgain that is fixed between 1 and 100.

Further, in the apparatus discussed above, the programmable gainamplifier cascade is preferably dynamically controlled by the computer;and the programmable gain amplifier achieves a gain of between at leastone of 1 and 30,000; and 1 and 100,000.

Further, the filter preferably provides band compensation; ananti-aliasing signal used for digital processing. In addition, thefilter is preferably configured to provide at least one of a Butterworthresponse, a Bessel response, a Chebychev Response, and an Ellipticresponse. Moreover, the filter is between 1st and 8th order, at leastone of passive and active, at least one of a continuous time filter anda switched capacitor, and implemented as a digital filter.

Further, the notch filter is preferably designed for at least one of a60 Hz cut-off and a 50 Hz cut-off, at least one of an active filter anda passive filter, and at least on of a continuous time filter, digitalfilter and a switched capacitor filter.

Further, for the apparatus discussed above, the computer is configuredto estimate the DC noise and to utilize a feedback control scheme forcanceling the DC noise; measures the amplified signal via ananalog-to-digital Converter; measures the amplified signal via a voltagecomparator; controls a DC reference generator for subtracting an initialinput DC offset voltage using a closed loop feedback scheme; controls atleast one of an audio alarm, visual display, machine interlock, andradio transmitter; generates a DC level via Digital-to-Analog Converterfor subtracting an input DC offset voltage with a closed loop feedbackscheme; generates a DC level via a buffered digital potentiometer forsubtracting an input DC offset voltage with a closed loop feedbackscheme; generates a DC level via pulse width modulation for subtractingan input DC offset voltage with a closed loop feedback scheme; andprovides various digital and analog outputs to control the componentsthat comprise the apparatus.

Moreover, for the apparatus discussed above, the analog output may rangebetween at least one of 0 and 5 volts, 0 and 1 volts, 0 and 10 volts, 0and 12 volts, 0 and 3.3 volts, and 0 and 24 volts; and the apparatus ispowered by a DC source of at least one of 5 Volts, 3.7 Volts, 7.4 Volts,3.3 Volts, 9 Volts, 12 Volts, 24 Volts, 110 Volts, and 220 Volts.

The flow diagram shown in FIG. 2 illustrates the method by which the MCUcontrols a system for enhancing electronic signals from a photodetector.Step 21 of FIG. 2 is directed to an initial step of powering orresetting the system. After power-up or when reset, the MCU initializesthe hardware of the system, as shown in step 22. In particular, step 22at least comprises setting the output voltages of the DACs to 0V;setting the PGA gain to zero; initializing the ADCs; and setting thecorner frequencies of the filters.

Next, in step 23, the MCU runs the Coarse Sample and Subtract Loop. Inparticular, in step 23A the MCU reads the voltage level of the outputsignal of the filter blocks, via the ADC value. Step 23B determineswhether the voltage level is above a predetermined coarse thresholdvoltage level.

If the voltage level of the output signal of the filter blocks is abovethe predetermined coarse threshold voltage level (i.e., “YES” output for23B), then a coarse adjustment is made in step 23C where the MCUincrements coarse DAC voltage. Step 23C has the effect of subtractingthe incremented voltage from the output signal. Steps 23A, 23B and 23Care repeated until the DC component of the input signal has beencanceled to within the predetermined coarse threshold voltage level.When the voltage level is within the predetermined coarse thresholdvoltage level (i.e., “NO” output for 23B), the method continues to theFine Sample and Subtract Loop 24, as shown in FIG. 2.

In the Fine Sample and Subtract Loop 24 of FIG. 2, the entire process ofsampling the filtered output voltage and incrementing a DAC is repeatedwith the fine DAC adjustment. In particular, in step 24A the MCU readsthe voltage level of the output signal of the filter blocks, via the ADCvalue. Step 24B determines whether the voltage level is above apredetermined fine threshold voltage level.

If the voltage level of the output signal of the filter blocks is abovethe predetermined fine threshold voltage level (i.e., “YES” output for24B) then a fine adjustment is made in step 24C, where the MCUincrements fine DAC voltage. Step 24C has the effect of subtracting theincremented voltage from the output signal. Steps 24A, 24B and 24C arerepeated until the DC component of the input signal has been canceled towithin the predetermined fine threshold voltage level. Until the outputvoltage is less than the predetermined fine threshold voltage. There maybe as many successively finer DAC adjustments and threshold voltages asa specific application demands. When the voltage level is within thepredetermined fine threshold voltage level (i.e., “NO” output for 24B),the method continues to the Sampling/Processing section 25, as shown inFIG. 2.

Step 25 of FIG. 2 shows the Sampling/Processing Loop 25. After the DCcancellation of step 23 and step 24 is completed, the MCU willcontinuously sample the filtered signal via the ADC in step 25A.Sampling is performed by the ADC at regular time intervals in accordancewith the Nyquist sampling theorem (i.e., at least two (2) times thehighest frequency component). The MCU may then implement any number ofdigital filtering, pattern recognition, or predictive control algorithmsin the Digital Filtering and Signal Processing functions of step 25B.Non-limiting examples of such algorithms include Proportional Integral,Least Mean Square or Kalman Filter.

In step 26, the MCU outputs the results via at least one of an outputDAC, serial output port, parallel output port, USB output port and RadioLink before continuously repeating the Sampling/Processing Loop 25. TheMCU may also control specific output devices such as an audio alarm,visual display, machine interlock, radio transmitter, or any otherelectrically controlled device. The Sampling/Processing Loop 25 willcontinue until either the device is powered down or reset by the user orby the MCU in response to a preprogrammed condition.

The foregoing description illustrates and describes the presentinvention. Additionally, the disclosure shows and describes only thepreferred embodiments of the invention, but as mentioned above, it is tobe understood that the invention is capable of use in various othercombinations, modifications, and environments and is capable of changesor modifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings and/or skill or knowledgeof the relevant art. The embodiments described hereinabove are furtherintended to explain best modes known of practicing the invention and toenable others skilled in the art to utilize the invention in such, orother embodiments and with the various modifications required by theparticular applications or uses of the invention. Accordingly, thedescription is not intended to limit the invention to the form orapplication disclosed herein. Also, it is intended that the appendedclaims be construed to include alternative embodiments.

1. A method for enhancing electronic signals from a photodetector comprising: at least one of starting and resetting the photodetector; initializing digital-to-analog converters (DACs), analog-to-digital converters (ADCs) programmable gain amplifiers (PGAs) and filter parameters; subtracting a voltage increment from the output of a filter until the output of the filter is at least one of less than and equal to a predetermined coarse threshold voltage; subtracting a voltage increment from the output of the filter until the output of the filter is at least one of less than and equal to a predetermined fine threshold voltage; filtering and signal processing the output of the filter; and outputting the filtered and signal processed output of the filter until receiving at least one of a power down and reset command.
 2. The method of claim 1, wherein initializing further comprises setting the output voltages of the DACs to 0V; setting the PGA gain to zero; initializing the ADCs; and setting the corner frequencies of the filters.
 3. The method of claim 1, wherein subtracting a voltage increment in accordance with the predetermined coarse threshold voltage further comprises reading the ADC value, determining when the ADC value is at least one of less than and equal to the coarse threshold, and incrementing the coarse DAC voltage when the ADC value is at least one of less than and equal to the coarse threshold.
 4. The method of claim 1, wherein subtracting a voltage increment in accordance with the predetermined fine threshold voltage further comprises reading the ADC value, determining when the ADC value is at least one of less than and equal to the fine threshold, and incrementing the fine DAC voltage when the ADC value is at least one of less than and equal to the fine threshold.
 5. The method of claim 1, wherein sampling by the ADC is performed at regular time intervals of at least two (2) times a highest frequency component in the filter output.
 6. The method of claim 1, wherein the filtering and signal processing includes at least one of digital filtering, pattern recognition, or predictive control algorithms.
 7. The method of claim 2, wherein the filtering and signal processing further includes at least one of Proportional Integral, Least Mean Square or Kalman Filtering.
 8. The method of claim 2, wherein outputting the filtered and signal processed output is provided via at least one of an output DAC, serial output port, parallel output port, USB output port and radio link. 